Radiation is highly effective in inducing apoptosis in skin-infiltrating T-cells and therefore exerts beneficial effects in patients with T cell-mediated skin diseases. However, for sufficient clinical efficacy, the sensitivity of (malignant) T cells for radiation may be enhanced. After UV-exposure of lymphoma cells, an accumulation of cells in the G1 phase and an increase of the apoptotic cell fraction was observed, which was augmented by treatment with 2-AP, a G1 checkpoint inhibitor (Takemura T et al., Apoptosis. 4(4), 245-53 (1999)). This study also showed that increased expression levels of p53 in G1 phase were linked to increased sensitivity to UV-irradiation-induced cell death. Increased radiosensitivity due to elevated p53 expression in the G1 phase was supported by a number of other studies (Cuddihy A R et al., Cancer Metastasis Rev. 23(3-4), 237-57 (2004), McIlwrath A J et al., Cancer Res. 54(14), 3718-3722 (1994), Bohnke A et al., Int J Radiat Biol. 80(1), 53-63 (2004), Nagasawa M et al., Oncogene. 20(23), 2889-99 (2001)). Furthermore, squamous cell carcinoma cells were shown to be sensitized for radiation through the induction of a G1 arrest by the anti-EGFR antibody, C225 (Huang S M et al., Clin Cancer Res. 6(6), 2166-74 (2000)). Tumor cells in a G1-like quiescent phase were also found to be more sensitive to radiation than proliferating cells (Ng C E et al., Br J Cancer. 56(3), 301-307 (1987)).
Upregulation of p53 has been shown to be related to inhibition of phosphatidylinositol 3-kinase (PI3K), an important signal pathway in T cells (Grandage V L et al., Leukemia. 19(4), 586-94 (2005)). As the inhibitory adapter molecule SHIP-1 is an inhibitor of PI3K (Horn S et al., Leukemia. 18(11), 1839-49 (2004)), activation of SHIP-1 would therefore lead to upregulation of p53
Dok-1 and SHIP-1 are so called “inhibitory adaptor molecules” expressed in hematopoietic cells, which serve to attenuate signaling and thereby prevent inappropriate cellular activation (Veillette A et al., 55(2), 301-8 (1988)). Phosphorylation of Dok-1 triggers interaction with SHIP-1, which leads to negative regulation of the PI3K protein kinase B (PKB)/Akt pathway. Over-expression of Dok-1 in B cells has been shown to cause an increase in the expression of the cell cycle inhibitor p21WAF1/Cip1, a decreased cyclin D2 expression, and a decreased expression of the anti-apoptotic protein bcl-XL (Yamakawa N et al., EMBO J. 21(7), 1684-94 (2002)). The increase of the G1/S inhibitor p21 and decrease of the G1 cyclin D would imply an extension or arrest in the G1 phase upon Dok-1 activation.
Artificially induced expression of SHIP-1, by restoration of SHIP-1 expression in endogenously SHIP-1-deficient Jurkat T cells, was shown to increase the transit time through the G1 phase. This extension of the G1 phase was associated with increased stability of cell cycle inhibitor p27Kip1 (Horn, 2004 supra). The inhibiting influence of SHIP activation on progression through the G1 phase was confirmed by investigation of the more ubiquitously expressed homologue of SHIP-1, SHIP-2, which is also expressed in T cells (Bruyns et al., Biol. Chem. 380(7-8), 969-74 (1999)). Overexpression of SHIP-2 in glioblastoma cells inhibited the PI3K protein kinase B (PKB) pathway and caused a cell cycle arrest in G1, which was also associated with increased stability of cell cycle inhibitor p27Kip1 (Taylor V et al., Mol Cell Biol. 20(18), 6860-6871 (2000)).
A variety of therapies that induce DNA damage, including UVB radiation, psoralen and UVA (PUVA) therapy, ionising radiation, electron beam, and x-ray (Kacinski et al., Ann N Y Acad. Sci. 941, 194-199 (2001)), and photopheresis (extracorporeal circulation of the blood with UVA and psoralen exposure) (Baron et al., Dermatol Ther. 16(4), 303-310 (2003)) have been indicated for CTCL.
PUVA is a highly effective treatment for cutaneous disease caused by skin infiltration with normal or neoplastic T-lymphocytes. It was reported that T-lymphocytes were greater than 50 fold more sensitive to cytotoxic effect of PUVA than other skin-resident cells such as keratinocytes (Johnson et al., Photochem Photobiol. 63(5), 566-571 (1996). A sub-G1 DNA peak indicated that cell death occurred by apoptosis and PUVA treatment markedly slowed cell cycle progression, eventually producing cell cycle arrest and apoptotic entry (Johnson R et al., Photochem Photobiol. 63(5), 566-571 (1996)). It has also been shown that PUVA leads to cross-links between DNA strands in the irradiated skin and that the damaged DNA will activate DNA repair mechanisms and cells are arrested in the G2 phase of the cell cycle in epidermal cells (Hashimoto Y et al., J Dermatol Sci. 10(1), 16-24 (1995) or fibroblasts (Ma W et al., Exp Dermatol. 12(5), 629-37 (2003).